PROJECT SUMMARY/ABSTRACT Bone morphogenetic proteins (BMPs) belong to the transforming growth factor (TGF) superfamily of ligands and the BMP signaling pathway plays roles in multiple developmental and homeostatic processes. Malfunction of the pathway causes many somatic and hereditary disorders in humans, including cardiovascular diseases and cancer. Thus mechanisms must exist to ensure proper spatiotemporal control of BMP signaling in the right cellular context. We are using the free-living nematode, C. elegans, as a model system to dissect the molecular mechanism modulating BMP signaling in vivo. C. elegans, with its wealth of genetic and molecular tools and the availability of the entire lineage, provides an excellent model system to study the functions and modulation of BMP signaling during the development of an intact organism at single cell resolution. We have developed a novel and efficient genetic screen that has allowed the identification of factors specifically modulating the BMP-like Sma/Mab signaling pathway in C. elegans. We have demonstrated that the single C. elegans RGM protein DRAG-1 acts at the ligand-receptor level to positively modulate Sma/Mab signaling. We have also discovered that the C. elegans neogenin homolog UNC-40 functions by directly binding to DRAG-1 to promote Sma/Mab signaling and that this function is separable from its function in axon and cell migration. Our screen has also uncovered a role for a conserved tetraspanin molecule TSP-21 and glycosphingolipids (GSLs) in promoting Sma/Mab signaling, providing the first in vivo evidence for the involvement of tetraspanins-enriched membrane microdomains in modulating TGF signaling. Finally, our mutant screen also indicated the presence of additional novel factors functioning in the Sma/Mab pathway. Further identification of the corresponding proteins and mechanistic dissection on how these proteins function in modulating Sma/Mab signaling will provide important insights into the molecular mechanisms involved in regulating BMP signaling in developing animals in vivo. They may also provide potential therapeutic targets for the different diseases caused by mutations in the BMP pathway.